I wouldnt want a single player version of ffxi, that would be really boring, ffxi has always been more fun with other players, even today. Plus i dont think it's possible as most of the code outside of assets is server side so converting that to single player would be pretty tough and offer little return for se.

The monthly sub is continuing as reducing it will not bring in enough players to gain more money for se, and the game isn't really set up for things like micro transactions and that will provbably lose them more players at this point when the focus is keeping the old player base.

In a perfect world when FFXI dies they will release the code or at least make some kind of toolkit to allow people to make there own servers but i don't see how se would make money off that so realistically it wont happen.

What is the most likely scenario is the mobile version will get released at some point, and SE will hope everyone will go play that and eventually release the mobile version on other platforms like the Switch, at that point the original ffxi shuts down and SE still has a numbered ff title still around. But that's a way off, FFXI in it's current form has at least another 5 years left i would say.